The present invention relates to improved optical phase retardation films.
Direct view optical display devices based on liquid crystal materials have been extensively developed over the past two decades. One embodiment which is exemplary of such devices is the twisted nematic (TN) liquid crystal display device. In this embodiment, a nematic liquid crystal medium is sandwiched between substrates which are treated so as to cause spontaneous alignment of liquid crystal molecules parallel to the plane of the substrate. If the two substrates are oriented so that alignment at each substrate differs by 90 degrees, then the liquid crystal molecules will undergo a 90 degree orientation change throughout the thickness of the medium. For reasonable spacing of the substrates (typically about 5 micrometers), this configuration has the property of rotating the polarization of light incident normal to the plane of the substrates by 90 degrees. If an electric potential is applied between the substrates (typically a few volts), then the order of the liquid crystal molecules is altered. In the presence of the potential, the molecules will tend to align perpendicular to the substrate and the 90 degree rotation will be destroyed. Thus the polarization of light incident normal to the surface of the substrate will be unaltered in the presence of the electric potential. Based on these principles, direct view twisted nematic (TN) liquid crystal devices may be constructed which are normally black (NB) in the absence of the potential or which are normally white (NW) in the absence of the potential.
As an example, a typical normally white display consists of a TN liquid crystal cell as described above fitted with polarizing elements on either side of the cell so that unpolarized light incident normal to the plane of the device is linearly polarized as it enters the device. The polarization is rotated by 90 degrees as the light transverses the cell. The light is then transmitted by the second polarizing element which is oriented at 90 degrees to the first polarizing element. Thus in the absence of an electric potential, light incident normal to the device is transmitted through the structure. When an electric potential is applied between the substrates, the media no longer rotates the polarization by 90 degrees. Thus light which is incident normal to the structure is rejected by the second polarizer and not transmitted. In this way, image information contained in the pattern of applied electric potential is presented as a decrease in light as seen by the viewer. This is the operating principle of a simple twisted nematic normally white (NW) display.
A key property of optical display devices is optical contrast, that is, the ratio of the light intensity in the "light on" state (for a NW display, the state with no potential applied) to the light intensity in the "light off" state (for a NW display, the state with the maximum potential applied). High contrast is important since the higher the contrast, the more readily the viewer will be able to distinguish between the two states. For viewing normal to the viewing screen, TN liquid crystal devices are capable of very high contrast (200:1 or more depending on the quality of the polarizers and other optical factors). However for viewing at an angle other than normal to the viewing screen, the contrast of simple TN devices is known to degrade significantly. For a normally white (NW) display, this is primarily due to the fact that rays of light which pass at an angle through the cell in the "light off" state (potential applied) experience a conversion from linear to elliptical polarization due to the greater refractive index perpendicular to the substrates that is n.sub.z &gt;n.sub.x, n.sub.y. This conversion is well known in the field of optics and depends on the magnitude of the birefringence (the change in refractive index, .delta.n); the thickness of the liquid crystal cell, d; and the incident angle. The elliptical character, i.e. components of both polarization directions, of the polarization allows light propagating at an angle other than normal to the viewing screen to escape through the second polarizer, and thus, lowers the ratio of light transmitted without the potential applied to that transmitted when the potential is applied.
Previous attempts to increase viewing angle, color purity, and contrast have involved the production of multi-domain liquid crystal cells (A. Lien et al., "Two-Domain TN-LCDs Fabricated by Parallel Fringe Field Method", SID 93 DIGEST, 269-271 (1993)) and the use of compensation cells (T. Nagatsuka et at., "Retardation Film for LCDs", Proceedings of the SID, 32/2, 125-131 (1991)), positive-birefringence films, negative-birefringence films (H. Ong, "Negative-Birefringence Film-Compensated Multi-Domain TN-LCDs with Improved Symmetrical Optical Performance", SID 93 DIGEST, 658-661 (1993)), laminates of multiple birefringence films (T. Nagatsuka et al., supra) and combinations of these and other techniques. No single technique has resolved any of these issues in a totally satisfactory manner, and the best results have been achieved with processes which involve difficulties in manufacture. A need for a simple process to enhance viewing angle both horizontally and vertically still exists. Additionally, a need exists for stability at a storage temperature of 125.degree. C., particularly in avionics applications.
"Phase retarder" films which can approximately compensate for this phenomenon by providing for enhanced contrast over a wider range of viewing angles are known. These films have the property that they convert elliptically polarized light to linearly polarized light and therefore compensate the polarization caused by the liquid crystals in the off axis direction. For a twisted nematic (TN) normally white (NW) device, these films are characterized by a negative birefringence, that is the refractive index for light polarized perpendicular to the plane of the film is less than that for light polarized parallel to the plane of the film. In this way, the film can compensate for the changes in polarization induced by the liquid crystal. These films are typically placed between the liquid crystal cell and at least one polarizer. The conversion depends upon the magnitude of the birefringence, .delta.N; the thickness, D; and the viewing angle. Thus, the mathematical condition for compensation is approximately -(.delta.n)d=(.delta.N)D wherein .delta.n is the birefringence of the liquid crystal, d is the thickness of the liquid crystal, .delta.N is the birefringence of the film, and D is the thickness of the film. For a typical NW liquid crystal display, .delta.n is about 0.06 and d is about 5 micrometers and thus the phase retarder requires typically (.delta.N)D about 0.3 micrometers.
A number of polymeric materials, which may be produced in film form, are taught to be useful as "phase retarder" films. For example, U.S. Pat. Nos. 4,385,806; 4,844,569, 5,061,042; 5,093,739; 5,121,238; 5,124,824; 5,138,474; 5,189,538; 5,245,456; and 5,249,071 teach that fluorine-containing resins, polyphenylene sulfide, polyvinyl butyral, cellulose butyrate, cellulose acetate, polyvinyl chloride, polyacrylonitrile, polystyrene, polyolefins such as polyethylene and polypropylene, polyetheramide, diallylcarbonate, polyamide, polyphenyleneoxide, acrylic ester polymers, methacrylic ester polymers, methacrylonitrile polymers, polysulphones, polyethylene naphthalates, polyether sulphones, polyether ether ketones, polyethylene terephthalate (PET) and blends, phenoxy ethers, norboranes, polycarbonate (PC), polymethyl methacrylate (PMMA) and blends, polystyrene blends with polymethyl methacrylate and with polycarbonates, aromatic polyesters, amorphous polyesters, polyvinylidene fluoride blends with PMMA, polyvinyl alcohol, polyimides, and cellulose diacetates are useful as phase retarder films.
U.S. Pat. No. 5,061,042 teaches that uniaxially stretched thermoplastic films are useful phase retarders and that the films may be uniaxially stretched by preheating, stretching, and heat setting. As noted in U.S. Pat. No. 5,061,042, it is most desirable to have a retardation value relating to the refractive index difference n.sub.x -n.sub.y between 30 and 1200 nm. As will be explained more fully below, we have found that for some liquid crystal cells, it is desirable to have a retardation value relating to the index difference n.sub.x -n.sub.z and n.sub.y -n.sub.z between about 30 and about 500 nm.
U.S. Pat. No. 5,245,456 teaches that a film made of polycarbonate, polystyrene, poly(vinyl alcohol), cellulose acetate, polyester, polyarylate, or polyimide has refractive indices, n.sub.x and n.sub.y, in two directions parallel to the film plane and crossing each other at right angles and a refractive index, n.sub.z, in the direction of the thickness of the film wherein n.sub.x &gt;n.sub.z &gt;n.sub.y. However, this relation between the refractive indices of the phase retardation film will not correct the change in polarization caused by propagation of light through the liquid crystal medium at an angle other than normal for all available liquid crystal materials. As will be explained more fully below, for some liquid crystalline materials, a phase retardation film that combines birefringence with negative optical anisotropy, that is, a film with refractive indices described by n.sub.x .gtoreq.n.sub.y &gt;n.sub.z, will more effectively compensate the birefringence with positive optical anisotropy of the liquid crystalline material and this will provide a liquid crystal display device with increased viewing angle and higher contrast.
PET and PC materials, which have been extensively utilized commercially, are of particular importance as phase retarder films because they have excellent optical and physical properties. Although PET and PC films have been widely applied as phase retarder films, they are severely deficient in several properties including dispersion (variation in optical properties such as .delta.N with wavelength), poor optical transparency, poor weathering properties (degradation of optical properties upon exposure to ultraviolet light), and low use temperatures. Other known films are also characterized by one or more of these deficiencies or are not useful for other reasons.
As such, the need exists in the art for an optical phase retarder film having low dispersion, a high degree of optical clarity, excellent weatherability, and optical properties which are stable over a wide range of use temperatures.